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Adding md5sums to iso images
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There are 512 bytes reserved for application specific use in the iso header
at offset 0x373 (cf. application_data in struct iso_primary_descriptor in
/usr/include/linux/iso_fs.h). We're free to do anything with it.

mkisofs fills it with spaces (512 x ' '), there's no mkisofs command line
option to set the field to some other value.

'tagmedia' calculates the md5sum (which is the same as running md5sum on the
whole iso) and puts a line like 'md5sum=d41d8cd98f00b204e9800998ecf8427e'
into the field.

'checkmedia' calculates the md5sum but assumes to be spaces in the range
0x8373-0x8572 of the iso image (iso header starts at 0x8000) and compares
the result against the stored md5sum.